Carbon-carbon composites are widely used for aircraft brake friction materials. Carbon-carbon is attractive because it is lightweight and can operate at very high temperatures, and because it can, pound for pound, absorb a great deal of aircraft energy and convert it to heat. A major drawback with the use of carbon-carbon for this application is the high cost of raw material used to make the parts. Expensive carbon fiber is a significant component; sometimes up to 45% fiber is used in the composite. Fiber costs can often be the single largest contributor to the cost of making a friction material. Another drawback is that manufacture of carbon-carbon is a time-consuming process. The overall process for making a carbon brake disk is measured in weeks, and even months. Long cycle times are undesirable in a modern manufacturing environment. It is highly desirable to provide a process that has a reduced cost and shortened cycle time for making a carbon-carbon composite.
The inventions disclosed herein address those major drawbacks of manufacturing carbon-carbon composites: cost and cycle time. As pointed out by Hager and Lake ("Novel Hybrid Composites Based on Carbon Foams", Mat. Res. Soc. Symp. Proc., Vol. 270, (1992), pp. 29-33), it is possible to create a reticulated carbon foam structure from mesophase pitch. This structure would have substantial fiber-like properties. The foam could subsequently be used to reinforce or form a composite which would behave in many respects like a carbon-fiber reinforced composite. By using a foam preform, instead of carbon fiber, the lower cost fiber precursor can be used, and the preform can be made in a single foaming step, instead of using a laborious process of manufacturing a needled carbon fabric, and needling a preform, or making fiber prepregs which are compacted by subsequent molding.
The use of foam preforms for the manufacture of friction material is disclosed by Tsang et al. in U.S. Pat. No. 4,537,823. However, it is advantageous to: 1) use typically a graphitizable carbon foam from a mesophase pitch, rather than a glassy carbon foam, 2) fill the void spaces with a carbonaceous material, rather than a polymeric material or a slurry, and 3) provide foams with a pore size less than 500 .mu.m to facilitate subsequent densification.
Carbon foams made from mesophase pitch have been disclosed in Mehta et al., "Graphitic Carbon Foams: Processing and Characterization", American Carbon Society, 21st Biennial Conference on Carbon, Buffalo, N.Y., Jun. 13-18, 1993. These foams were not densified because the foams were to be used for lightweight structural applications that did not require densification.
Also, foams of carbonaceous material have been known and methods of preparing them have been disclosed for absorption or filtration media and supports for catalysts, etc., and is generally made from polymeric precursors (thermosets and thermoplastics) which usually produce amorphous or non-crystalline carbons.